Liquid hydrocarbon fuels, and in particular gasoline, consist of a multitude of components having a wide range of boiling points and volatilities. During normal operation of a motor vehicle having an internal combustion engine, the gasoline in the vehicle's fuel tank produces fuel vapors. Currently, all gasoline powered automobiles are required to be equipped with fuel vapor recovery systems to trap the fuel vapors produced during operation before they can escape into the environment. Future regulations are likely to require vehicles to recover and store even the fuel vapors generated during vehicle refueling.
Hybrid motor vehicles typically consist of one or more electric traction motors that drive the wheels of the vehicle, storage batteries to supply electrical energy to the traction motor, and some sort of generator or alternator driven by the internal combustion engine to charge the batteries and/or to provide power for the traction motor. The combination of the engine and the generator/alternator, known as an auxiliary power unit (APU) or range extender, provides a means for converting the chemical energy of the engine's liquid fuel into electrical energy.
Hybrid vehicles utilizing volatile liquid fuels will probably also be required to have a vapor recovery system. Vapor recovery systems typically purge the fuel vapors from a storage canister into the engine every time the vehicle is started. Hybrid vehicles, however, experience unique problems in purging their vapor recovery systems. For example, if the vehicle batteries have a relatively large storage capacity, the APU usually need not be turned on every time the vehicle is used. Therefore, opportunities for vapor purging may occur only at long and irregular intervals. Under such conditions the vapor recovery system may become overloaded, and release fuel vapors into the atmosphere. A less than ideal means of purging the vapor recovery system is to start the APU at regular intervals, for the sole purpose of consuming the stored vapors, regardless of the need of the vehicle for electricity. Such extraneous start-ups, however, shorten engine life and produce unnecessary exhaust emissions.
In a conventional automobile, heat for the passenger compartment is extracted from engine coolant being circulated through a heater core, and is soon available after vehicle start-up. In a hybrid vehicle, APU engine start-up is independent from vehicle start-up, making engine heat an unreliable source for passenger heat on initial vehicle driveaway. Furthermore, the energy storage capability of current battery technology severely limits the practicality of electric passenger heaters because their large electrical energy requirement significantly diminishes the vehicle's electric only driving range capability.